diff options
authorLinus Torvalds <torvalds@linux-foundation.org>2017-08-27 13:55:12 -0700
committerLinus Torvalds <torvalds@linux-foundation.org>2017-08-27 13:55:12 -0700
commit3510ca20ece0150af6b10c77a74ff1b5c198e3e2 (patch)
parent0cc3b0ec23ce4c69e1e890ed2b8d2fa932b14aad (diff)
Minor page waitqueue cleanups
Tim Chen and Kan Liang have been battling a customer load that shows extremely long page wakeup lists. The cause seems to be constant NUMA migration of a hot page that is shared across a lot of threads, but the actual root cause for the exact behavior has not been found. Tim has a patch that batches the wait list traversal at wakeup time, so that we at least don't get long uninterruptible cases where we traverse and wake up thousands of processes and get nasty latency spikes. That is likely 4.14 material, but we're still discussing the page waitqueue specific parts of it. In the meantime, I've tried to look at making the page wait queues less expensive, and failing miserably. If you have thousands of threads waiting for the same page, it will be painful. We'll need to try to figure out the NUMA balancing issue some day, in addition to avoiding the excessive spinlock hold times. That said, having tried to rewrite the page wait queues, I can at least fix up some of the braindamage in the current situation. In particular: (a) we don't want to continue walking the page wait list if the bit we're waiting for already got set again (which seems to be one of the patterns of the bad load). That makes no progress and just causes pointless cache pollution chasing the pointers. (b) we don't want to put the non-locking waiters always on the front of the queue, and the locking waiters always on the back. Not only is that unfair, it means that we wake up thousands of reading threads that will just end up being blocked by the writer later anyway. Also add a comment about the layout of 'struct wait_page_key' - there is an external user of it in the cachefiles code that means that it has to match the layout of 'struct wait_bit_key' in the two first members. It so happens to match, because 'struct page *' and 'unsigned long *' end up having the same values simply because the page flags are the first member in struct page. Cc: Tim Chen <tim.c.chen@linux.intel.com> Cc: Kan Liang <kan.liang@intel.com> Cc: Mel Gorman <mgorman@techsingularity.net> Cc: Christopher Lameter <cl@linux.com> Cc: Andi Kleen <ak@linux.intel.com> Cc: Davidlohr Bueso <dave@stgolabs.net> Cc: Peter Zijlstra <peterz@infradead.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
2 files changed, 10 insertions, 8 deletions
diff --git a/kernel/sched/wait.c b/kernel/sched/wait.c
index 17f11c6b0a9f..d6afed6d0752 100644
--- a/kernel/sched/wait.c
+++ b/kernel/sched/wait.c
@@ -70,9 +70,10 @@ static void __wake_up_common(struct wait_queue_head *wq_head, unsigned int mode,
list_for_each_entry_safe(curr, next, &wq_head->head, entry) {
unsigned flags = curr->flags;
- if (curr->func(curr, mode, wake_flags, key) &&
- (flags & WQ_FLAG_EXCLUSIVE) && !--nr_exclusive)
+ int ret = curr->func(curr, mode, wake_flags, key);
+ if (ret < 0)
+ break;
+ if (ret && (flags & WQ_FLAG_EXCLUSIVE) && !--nr_exclusive)
diff --git a/mm/filemap.c b/mm/filemap.c
index a49702445ce0..baba290c276b 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -885,6 +885,7 @@ void __init pagecache_init(void)
+/* This has the same layout as wait_bit_key - see fs/cachefiles/rdwr.c */
struct wait_page_key {
struct page *page;
int bit_nr;
@@ -909,8 +910,10 @@ static int wake_page_function(wait_queue_entry_t *wait, unsigned mode, int sync,
if (wait_page->bit_nr != key->bit_nr)
return 0;
+ /* Stop walking if it's locked */
if (test_bit(key->bit_nr, &key->page->flags))
- return 0;
+ return -1;
return autoremove_wake_function(wait, mode, sync, key);
@@ -964,6 +967,7 @@ static inline int wait_on_page_bit_common(wait_queue_head_t *q,
int ret = 0;
+ wait->flags = lock ? WQ_FLAG_EXCLUSIVE : 0;
wait->func = wake_page_function;
wait_page.page = page;
wait_page.bit_nr = bit_nr;
@@ -972,10 +976,7 @@ static inline int wait_on_page_bit_common(wait_queue_head_t *q,
if (likely(list_empty(&wait->entry))) {
- if (lock)
- __add_wait_queue_entry_tail_exclusive(q, wait);
- else
- __add_wait_queue(q, wait);
+ __add_wait_queue_entry_tail(q, wait);